Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle- or large-sized devices, such as vehicles, use a middle- or large-sized battery module having a plurality of battery cells electrically connected with each other because high power and large capacity are necessary for the middle- or large-sized devices.
Preferably, the middle- or large-sized battery module is manufactured with small size and weight if possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle- or large-sized battery module. Especially, much interest is currently generated in the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the weight of the pouch-shaped battery is small, the manufacturing costs of the pouch-shaped battery are low, and it is easy to modify the shape of the pouch-shaped battery.
In order for the middle- or large-sized battery module to provide power and capacity required by a predetermined apparatus or device, it is necessary for the middle- or large-sized battery module to be constructed in a structure in which a plurality of battery cells are electrically connected in series to each other, and the battery cells are stable against an external force.
Also, the battery cells constituting the middle- or large-sized battery module are secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from the high-power, large-capacity secondary batteries during the charge and discharge of the batteries. If the heat, generated from the unit cells during the charge and discharge of the unit cells, is not effectively removed, the heat accumulates in the respective unit cells, with the result that the deterioration of the unit cells is accelerated. According to circumstances, the unit cells may catch fire or explode. For this reason, a cooling system is needed in a battery pack for vehicles, which is a high-power, large-capacity battery, to cool battery cells mounted in the battery pack.
In a middle- or large-sized battery pack including a plurality of battery cells, on the other hand, the deterioration in performance of some battery cells leads to the deterioration in performance of the entire battery pack. One of the main factors causing the nonuniformity of the performance is the nonuniformity of cooling between the battery cells. For this reason, it is required to provide a structure to secure the uniformity of cooling during the flow of a coolant.
Some conventional middle- or large-sized battery packs use a battery pack case constructed in a structure in which a coolant inlet port and a coolant outlet port are located at the upper part and the lower part of the battery pack case such that the coolant inlet port and a coolant outlet port are directed in opposite directions, and the top and bottom of a flow space extending from the coolant inlet port to the battery module are parallel to each other. In this structure, however, a relatively high coolant flux is introduced into flow channels defined between the battery cells adjacent to the coolant outlet port, whereas a relatively low coolant flux is introduced into flow channels defined between the battery cells adjacent to the coolant inlet port, with the result that it is difficult to achieve uniform cooling of the battery cells.
In connection with this matter, Korean Patent Application Publication No. 2006-0037600, No. 2006-0037601, and No. 2006-0037627 disclose a middle- or large-sized battery pack constructed in a structure in which an air guide plane is inclined downward to a side opposite to battery cells such that the air guide plane becomes closer to the battery cells with the increase in distance between the air guide plane and a coolant inlet port. Specifically, the air guide plane is inclined at an angle of 15 to 45 degrees to the side opposite to the battery cells, thereby restraining the occurrence of a phenomenon in which coolant flux is excessively introduced into flow channels defined between the battery cells adjacent to the coolant outlet port.
However, the inventors of the present application have found that the temperature deviation between the battery cells is great even in the above-described structure, with the result that it is not possible to achieve the temperature uniformity of a desired level.
Consequently, there is a high necessity for a technology to fundamentally solve the above-mentioned problems.